Was Man More Aquatic in the Past? Fifty Years After Alister Hardy - Waterside Hypotheses of Human Evolution

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For many investigators, the rôle of water in the evolution of the Hominini refers to the development of a number of anatomical and physiological features, which hominins are thought to share with water-adapted animals. However, in the last dozen years, there has been emphasis on other ways in which water, and the proximity to water, have been probable influences in hominin evolution. This chapter reviews each of five ways in which water has influenced or might have affected human evolution. This pentapartite analysis singles out water for drinking, for keeping cool, for global dispersal, as a basis for aquatic adaptations and for the ingesting of aquatic foods. In contrast with the heavy, earth-bound view of hominin evolution, which has predominated hitherto, an appeal is made here for students of hominin evolution to buoy up, lighten and leaven their strategy by adopting a far greater emphasis upon the role of water and waterways in hominin phylogeny, diversification, and dispersal from one water-girt milieu to others. Some evidence is adduced to show the value and potential of this course of action.

The ability to exploit and thrive on a wide variety of foodstuffs from diverse environments is a hallmark of Homo sapiens. Humans are particularly well adapted to exploit waterside environments, where they can forage in areas offering protection from both terrestrial and aquatic predators. Humans are able to walk, run, climb, wade, swim and dive, and our research indicates that the most parsimonious explanation for this combination of locomotor traits, and for Man's current anatomy, physiology, nutritional requirements and unique intellect is evolution in a littoral environment. This model is consistent with the location and presumed palaeoecologies of all early Homo fossils and artifacts, and could help explain the rapid dispersal of Homo in the early Pleistocene (2.56-0.78 million years ago (Ma)), the colonization of Australia and Indonesia in the middle Pleistocene (0.78-0.13 Ma), and the rapid dispersal of Homo sapiens in the late Pleistocene (0.13-0.012 Ma). Reliance on the aquatic food chain is also a facile method for providing consistently abundant brain-specific nutrition for all members of a group or society, thus facilitating the development of the technology and culture that is uniquely human.

For over 150 years the field of palaeo-anthropology has grappled with several problems of understanding human evolution, notably those explaining key differences between human beings and our most closely related species, the African great apes. The first difference to be explained, perhaps in terms of importance but certainly in terms of chronology, is our bipedality.

This chapter will review the models of hominin bipedal origins published to date, and categorize them, as was done by Rose [1], by the adaptive mechanism being suggested. In addition, it will propose a new evaluative framework against which each model may be assessed and compared. In this evaluation, published wading models appear to be among the strongest although they are among the least well reported in university-level text books, a discrepancy attributed here to their association with the so-called ‘ aquatic ape hypothesis’ (AAH). Despite their apparent strengths, published wading models do nevertheless contain weaknesses. This chapter addresses a few of those weaknesses either theoretically or through studies, such as one obtaining new empirical data comparing the energy efficiency of different bipedal gaits in water. Furthermore, a series of falsifiable predictions of the wading hypothesis are made about the postcranial anatomy of australopithecines.

The chapter concludes by proposing a specific timescale and ecological niche where such wading behavior could have provided a stable evolutionary scenario in early hominins that is compatible with the fossil record and other models of human evolution.

The great (orangutans, gorillas and chimpanzees) and lesser apes (siamangs and gibbons) are significantly different to monkeys, yet the evolution of the apes is rarely discussed in detail, especially from a human evolutionary perspective. Assuming that the early primates were arboreal and that human ancestors were semi-aquatic, human predecessors in the intermediary phase must have been aquarboreal, i.e., spent significant time in both trees (Latin arbor) and water (Latin aqua). Here we describe a number of independent indications that early apes – possibly as early as 20 Ma (million years ago) – were vertical aquarboreal frugi-omnivores in swamp forests.

Apes differ from monkeys in having a below-branch locomotion, with larger and broader bodies and thoraxes, very long arms that can easily be extended above the head, and tail loss. Whereas most mammals and monkeys predominantly move pronogradely (with horizontal spine and trunk), the remarkably humanlike lumbar vertebra of Morotopithecus suggests that by about 20 Ma the early apes were already orthograde (with a generally vertical spine). According to the palaeo-environmental data, the fossils of Mio-Pliocene apes typically lay in coastal and swamp forest sediments around the Tethys Sea (the ancient Mediterranean Sea). The Miocene (23.0 to 5.3 Ma) and the Pliocene (5.3 to 2.6 Ma) epochs were generally hotter and wetter than the Pleistocene Ice Ages (2.6 to 0.01 Ma). Recently, the highest population densities of orangutans as well as gorillas have been discovered in extremely hot and wet swamp forests.

Since all great apes can make and use tools, and most fossil great apes had thick enamel, the ancestral great ape diet in flooded forests might have included durophagy of hard-shelled foods (e.g., palm nuts or molluscs). Locomotor requirements for flooded forest dwelling could arguably have included a bigger body with vertical climbing abilities, including with arms overhead and arm-hanging. Lowland gorillas employ an orthograde posture and locomotion when they climb, wade through shallow swamps, and sit and feed in shallow water.

Compared to the skeletons of all other primates, including Homo sapiens, the crania and postcrania of Homo erectus were typically massive, displaying extremely thick bones with compact cortices and narrow medullary canals. Even outside the primate order, examples of animals displaying such massive bones are rare. Although this feature is sometimes seen as diagnostic of H. erectus, few convincing hypotheses have been put forward to explain its functional and adaptive significance.

Here, we present data showing that unusually heavy bones were a typical, although not exclusive nor indispensable, characteristic of H. erectus populations through the early, middle and late Pleistocene in areas of Asia, Africa and Europe. A comparative review of the occurrence of massive skeletons in other mammals suggests that they have an important buoyancy control function in shallow diving aquatic and semi-aquatic species, and are part of a set of adaptations that allow for the more efficient collection of slow, sessile and immobile foods such as aquatic vegetation and hard-shelled invertebrates. We therefore consider the possibility that part-time shoreline collection of aquatic foods might have been a typical element of the lifestyle of H. erectus populations. We discuss the alternative explanations for heavy bones from the literature, as well as apparent exceptions to the rule, such as thin-boned H. erectus and thick-boned Homo sapiens fossils. A review of the palaeo-ecological data shows that most, if not all, H. erectus fossils and tools are associated with water-dependent molluscs and large bodies of permanent water. Since fresh and salt water habitats have different densities, we hypothesize that in H. erectus as well as in some Homo sapiens populations, there might have been a positive correlation between massive bones and dwelling along sea or salt lake shores.

Missing in the literature to date is a concise description of the various scenarios proposing that human evolution was affected, to some extent, by selection from wading, swimming and diving through water. Most of it tends to focus on just one such scenario, first proposed by Hardy [1] and promoted by Morgan [2-6], which suggested that a more aquatic interlude was contemporaneous with, and probably caused, the split between Pan and Homo, and was followed by a U-turn back to a fully terrestrial life. Although theirs is still the most wellknown, it is not the only one. Other scenarios, for example that proposed by Verhaegen et al. [7-11], differ quite markedly in timescale, the proposed degree and mode of aquatic selection, and in terms of the evidence used in support. This Chapter reports more than ten such ideas and summarizes six aquatic scenarios and clarifies the differences between them. It also identifies a common thread between them, and uses it to propose a new label and definition for them.

Modern humans are generally considered to be fully terrestrial, yet display a range of activities involving breath-hold (apneic) diving, including sustained harvest diving, spear-fishing, recreational free-diving and competitive apnea for duration, distance or depth. Via harvest diving, involving repeated diving with half of the time submerged for several hours per day, groups in South East Asia obtain a considerable amount of catch. The physiological basis for such repeated diving involves i) conscious breath control, ii) an efficient diving response, diverting stored oxygen to the heart and brain, and iii) adequate thermal insulation. Another contribution to the human diving ability comes from the spleen, which - by ejecting extra red blood cells into circulation - can enhance blood gas storage and carbon dioxide buffering capacity, a response typically found in seals.

Most striking among human aquatic activities is competitive apnea, with records of a period of 11 min 35 s in duration, the distance of 265 m in underwater swimming with fins, and a depth of 124 m in deep-diving with fins. Without fins, the distance of 218 m and depth of 101 m have been achieved, performances in the range of marine mammals. This requires additional mechanisms to maximize gas storage, minimize energy expenditure, and enhance conscious tolerance to asphyxia, involving e.g. increase lung volume, baseline hematocrit and spleen volume, and means to cope with the increased pressure. While it takes both inherent predisposition and training to achieve such record results, most healthy humans can, after some practice, make voluntary apneas of 3-4 min, swim a distance of 50 m under water and reach depths of 20-30 m, which may be unique among terrestrial mammals.

Human superior harvest diving and competitive diving capacity may suggest a selective pressure for diving during some time period of human evolution.

Humans possess kidneys that are normally multi-pyramidal in their morphology, a characteristic that is unique to Homo sapiens amongst primates. While uni-pyramidal kidneys predominate in terrestrial mammals, kidneys with multiple medullary pyramids are nearly universal in marine mammals. In salt water environments, renal medullary pyramids appear to function as a means to increases the rate of salt and nitrogenous waste excretion by increasing the surface area between the cortex and medulla. While renal medullary pyramids seem to have no functional value in freshwater environments, most freshwater aquatic mammals with renal pyramids can be phylogenetically traced to either marine ancestors or aquatic ancestors that frequented marine environments. Terrestrial mammals with multi-pyramidal kidneys such as elephants, bears, and rhinoceroses also appear to have had semi-aquatic ancestors that frequented marine environments. However, the multi-pyramidal kidneys of the Bactrian camel and Arabian camel (dromedary) were apparently convergently evolved as adaptations to high salt consumption in xeric terrestrial environments where camels consume halophytic plants and drink water from brine pools with natural salinities higher than seawater. The numerous vestiges of aquatic adaptations in the human body in addition to the abundant distribution of corporeal salt excreting eccrine sweat glands and the excretion of salt tears in humans, strongly suggest that the multiple medullary pyramids of the human kidneys probably evolved as an adaptation to a coastal marine ecology rather than to a xeric terrestrial environment.

The aquatic ape hypothesis (AAH) offers the possibility to re-interpret several aspects of the human pre- and perinatal periods. We introduced the concept of birthing pools in the 1970s in order to treat labor pain, particularly lumbar pain in the middle of cervical dilation associated with failure to progress. We assumed that immersion in water at body temperature would be a way to reduce the level of stress hormones, facilitating the release of oxytocin. We first learned that the dilation of the cervix could progress dramatically in an aquatic environment before water immersion, and that it was associated with behavior suggestive of a reduction in neocortical control. The release of inhibitors of neocortical control in an aquatic environment was an opportunity to phrase new questions about the relationship of Homo sapiens with water. We also learned that occasionally some women did not want nor had the time to get out of the pool for the birth itself. They behaved in a way suggesting that, while in a particular state of consciousness, they knew that a birth under water was safe for the baby. The origin of this knowledge, and the strong attraction towards water that some women experience during labor, should be looked at in the light of the AAH.

By raising questions about the development of the human brain from an evolutionary perspective, the AAH enables to pose similar questions from an ontogenetic perspective. Today the focus is on the needs in iodine, docosahexaenoic acid, vitamin D, and other nutrients that are essential for brain development, and that happen to be abundant in seafood. This led us to reconsider (pre-)eclampsia as a multi-factorial syndrome, related to a maternalfetal conflict, whereby inadequate maternal nutrition is prioritized as a factor that can independently increase the probability of conflict, challenging the current belief that reduced utero-placental perfusion is the unique pathophysiological process in this human pregnancy disease. In other words, we present (pre-)eclampsia as the price some humans have to pay for having a large brain, while the specific nutritional needs are not ideally satisfied.

Other puzzling and unexplained human particularities in the perinatal period, such as human neonatal vernix caseosa and of the absence of human maternal placentophagy, can be re-interpreted in the light of the AAH.

Adaptability of the eye is a key feature in a semi-aquatic mammal and several optical and physiological strategies can be used to allow functioning of the eye in the two media, i.e., air and water. Human eyes are considered to be adapted to vision in air as more than two-thirds of the refractive power is derived from the curved cornea, an effect that is lost under water. It was observed that children of Sea Nomad groups in South East Asia appeared to have much better underwater vision than expected, allowing efficient collection of small shells from a non-contrasting background without visual aids. Studies on the visual acuity of such groups were carried out, followed by studies to reveal how the observed adaptability of the eye was achieved. Standardized optical methods were adopted to field conditions and used to reveal how the Sea Nomad children see under water. Results showed a high adaptability of the human eye to the underwater environment, with the visual acuity of the Sea Nomad children being twice that found in a European control group. Training in non-diving children was found to evoke the same adaptive responses as those observed in Sea Nomads. The mechanisms responsible for this superior underwater vision were heavy accommodation and concurrent pupil constriction, features previously observed in semi-aquatic mammals and birds. This may be an interesting example of convergent evolution. The human eye proved to be flexible and adaptable enough to function under water with an uncompromised function in air. An explanation for this surprising adaptation in a terrestrial mammal could be that it has evolved during a phase with selective pressure for foraging under water.

Many human physiological and behavioral features could possibly be explained as semi-aquatic adaptations in the remote past. However, aspects of human perception and cognition have rarely been considered in this light. In this chapter, human color vision will be discussed at two levels.

At the physiological level, visual pigments of retinal cone/rod cells, being essential to color vision, are compared among humans, their closest primate relatives, and terrestrial as well as aquatic mammals. Also the cause of human color blindness is discussed.

At the cultural level, the mystery of ‘fuzzy’ color terms like grue (green-or-blue) in many world languages is discussed, and we propose a new model based on two arguments: each color term actually corresponds to a naturally occurring color, and the ‘ fuzzy’ terms were produced in a semi-aquatic primitive life since the dawn of human language.

In this paper we present comparative data, suggesting that the various elements of human speech evolved at different times, and originally had different functions. Recent work by Nishimura [1-6] shows that what is commonly known as the laryngeal descent actually evolved in a mosaic way in minimally two steps: (a) a descent of the thyroid cartilage (Adam's apple) relative to the hyoid (tongue bone), a descent which is also seen in non-human hominoids, and (b) a descent of the hyoid bone relative to the palate, which is less obvious in non-human hominoids, and which is accentuated by the absence of prognathism in the short and flat human face. Comparisons with other animals suggest that (a) the first descent might be associated with loud and/or varied sound production, and that (b) the second might be part of an adaptation to eating seafoods such as shell fish, which can be sucked into the mouth and swallowed without chewing, even under water. We argue that the origin of human speech is based on different pre-adaptations that were present in human ancestors, such as (a) sound production adaptations related to the descent of the thyroid cartilage associated with the territorial calls of apes, (b) transformation of the oral and dentitional anatomy including the descent of the hyoid, associated with reduced biting and chewing, and (c) diving adaptations, leading to voluntary control of the airway entrances and voluntary breath control. Whereas chimpanzee ancestors became frugivores in tropical forests after they split from human ancestors about 5 Ma (million years ago), human ancestors became littoral omnivores. This might help explain why chimpanzees did not evolve language skills, why human language is a relatively recent phenomenon, and why it is so strongly dependent upon the availability of voluntary breath control, not seen in other hominoids, but clearly present in diving mammals.

This paper gives an overview of the beginnings and the scientific acceptance of the so-called aquatic ape hypothesis of human evolution (AAH), especially the work of Alister Hardy and Elaine Morgan. In 1960, marine biologist Sir Alister Hardy, in a notorious article in The New Scientist, entitled Was Man more aquatic in the past?, suggested that “a branch of the primitive ape-stock was forced by competition from life in the trees to feed on the sea shores, and to hunt for food, shell fish, sea urchins, etc., in the shallow waters off the coast … ”, and that “these semiaquatic creatures were soon wading into deeper water, and eventually began to swim and dive for food at even greater depths.” [1]. His hypothesis was based on observations such as human swimming and diving skills, our furlessness and subcutaneous fat tissues, and our streamlined shape “compared with the clumsy form of the ape.” Hardy's littoral hypothesis was brought into the attention of the great public by the books and papers of tv writer Elaine Morgan, beginning with her bestseller The descent of woman [2], and followed by The aquatic ape [3], The scars of evolution [4] and The aquatic ape hypothesis [5], in which she discussed the waterside theory in much greater detail, e.g., with regard to the origin of human bipedality and laryngeal descent and in which she made comparisons to the fossil swamp ape Oreopithecus and to living animals such as the mangrove-dwelling proboscis monkeys (Nasalis larvatus) and the arguably ex-aquatic elephants. This paper further discusses relevant work of Desmond Morris and Christian de Muizon, as well as diverse reactions of palaeo-anthropologists, such as professor Phillip Tobias's dismissal of the savannah hypothesis.

Science theory argues that all ideas are contextualized in their disciplines and also reflect historical and current cultural and social values. This chapter looks at the origins of the aquatic ape hypothesis (AAH) and its development over fifty years, focusing on the particular contributions of Elaine Morgan. The hypothesis had its genesis in a gendered debate on human difference and has connections with anthropology, primatology, palaeontology and other disciplines. More radically, it has a complex relationship with scientific method, gene-centered neo- Darwinism and theories that advocate multiple agents of biological change. Analysis of the AAH reveals much about the history of various genres in science writing, and constructions of scientific authority and knowledge.

Thus far, there has been no challenge to Langdon's 1997 critique [1]of the aquatic ape hypothesis (AAH), despite its having a number of weaknesses. The paper lacks scholarliness as it does not draw upon the one published scientific investigation into the plausibility of the AAH in the literature, i.e., that byRoede et al. [2]. Langdon's summary of “anatomical evidence for the AAH” seems to have been directed against an exaggerated interpretation of Alister Hardy's hypothesis that humans were “more aquatic in the past” [3]. Most of the critique was based on cursory and superficial comparisons with fully aquatic mammals, such as cetaceans, rather than considering whether human ancestors could have been more aquatic than those of apes. Even on this basis, Langdon considered eleven out of twenty-six traits to be “possible aquatic adaptations” or “consistent with the AAH”.

It is argued here that none of the specific hypotheses of the AAH have yet been refuted. Instead, what appears to have happened, is that individuals have been left to interpret certain ambiguities in arguments put forward by proponents of the hypothesis in their own way and then reject, or accept it on that basis. More than a decade later, significant new evidence has emerged, and other AAH-based models have been published, which demand serious reconsideration.

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